Processes for producing thicker gage products of niobium microalloyed steel

ABSTRACT

A process for controlling austenite grain size in austenite processing through nano-scale precipitate engineering of TiN—NbC composites to produce thicker gage product of niobium microalloyed steel includes controlling the base chemical composition of a steel product to include 0.003-0.004 wt. percent nitrogen, 0.012-0.015 wt. percent titanium, 0.03-0.07 wt. percent carbon, and 0.07-0.15 wt. percent nobium; lowering the temperature of roughening to end the roughening operation in the temperature range of from about 980° C. to 1030° C.; retaining greater than about 0.03 wt. percent niobium in solution in the matrix by rapid cooling of the product to enter the finish rolling operation below the temperature of no recrystallization, with an austenite grain size of about 30 microns; and applying reduced rolling reduction in the finish rolling operation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to austenite grain size control by preventinggrain coarsening of austenite in upstream processing of niobiummicroalloyed steel in order to produce thicker gage products withexcellent drop weight tear test (DWTT) toughness (as measured inaccordance with API RP 5L3 (Apr. 1, 1996)). In current technologicalpractice, there is no intentional control measure to prevent graincoarsening of austenite before entry to finish rolling. As aconsequence, heavy rolling reductions are applied to coarse grainedaustenite during finish rolling in order to increase the surface tovolume ratio by geometric means through heavily pancaked austenite.Heavy rolling reductions applied in the finishing mill, oftenapproaching limits of mill loading, inevitably limit thickness of thefinal product. In order to improve the safety and efficiency of thetransport of natural gas and oil through the pipe lines, there is agrowing demand for thicker gage pipes, particularly for deep offshoreprojects.

This invention targets austenite grain size control upstream i.e., athigh temperatures in order to produce thicker gage product. Thisinvention utilizes the formation of nano-scale TiN—NbC compositeprecipitates to pin austenite grain boundaries and prevent them fromcoarsening at high temperatures (>980° C.) so that less rollingreduction and pancaking of austenite is required to obtain targetproperties of high strength and excellent toughness at low temperatureas measured by ductile to brittle transition temperature and percentageshear in drop weight tear tests. There are additional benefits relatedto reduced texture related anisotropy of properties and improved ductilefracture arrestability of pipes in service resulting from lessthermo-mechanical rolling reduction during finish rolling.

2. Description of Related Art

Although austenite grain size at the end of roughing is fine (<20microns), significant grain coarsening of austenite occurs subsequent tothe end of roughing, which must be prevented from occurring in order toproduce a thicker gage product. If the austenite grain size is coarse,it is feasible to apply heavy rolling reductions to “pancake” theaustenite and increase the surface to volume ratio of austenite, whichincreases the nucleation sites for ferrite upon transformation. To theextent that grain coarsening is controlled by a diffusion mechanismwhich is dependent on time and temperature, rapid thermal cooling shoulddecrease the kinetics of diffusion for grain coarsening. However, it hasnot been found feasible to adequately cool the center of thick transferbars to prevent grain coarsening. Nevertheless, accelerated coolingupstream is still beneficial to avoid rolling in the partialrecrystallization regime and avoid depletion of solute niobium byexcessive growth of niobium carbide precipitates upstream. Therefore,there is a need to develop alternative strategies to prevent graincoarsening. Although solute niobium retards grain coarsening byretarding boundary mobility, the magnitude of solute drag on boundarymobility is weak at high temperatures. As a consequence, even thoughmost of the niobium is available as solute in conventional processing,it is not found to be effective in preventing grain coarsening. Thus, anobject of the present invention is to prevent grain coarsening ofaustenite grains by metallurgical means, through pinning austenite grainboundaries by second phase particles using the Zener pinning mechanism.The use of TiN particles to pin austenite grain boundaries by the Zenerpinning mechanism is well established and has been disclosed in priorpatents (see, for example, U.S. Pat. No. 6,899,773; U.S. Pat. No.6,183,573; and U.S. Pat. No. 5,900,075). While these patents identifyconditions under which high number density of TiN precipitates can bepromoted, the limiting austenite size achievable by TiN alone istypically 60 to 80 microns in the high temperature window of processing.NbC is sluggish to nucleate by itself and is aided by dislocationsgenerated by deformation to promote strain induced nucleation of NbC,which is associated with large undercooling. Strain inducedprecipitation of NbC is used in controlled rolling of microalloyingtechnology, where strain induced precipitation of NbC is used to pinaustenite grain boundaries during thermo-mechanical controlled rollingduring the low temperature window of processing. However, by promotingepitaxial growth of NbC on pre-existing TiN in accordance with thepresent invention, TiN—NbC composite precipitates are obtained withnegligible undercooling in the high temperature window at the end ofroughing. These nano-scale TiN—NbC composite precipitates are availableto pin austenite grains at the end of roughing and limit austenite grainsize to under about 30 microns on entry to finish rolling, which isessential to produce thicker gage in line pipe grades.

Accordingly, it is an object of the invention to reduce the need forlarge rolling reductions and heavy pancaking during finish rolling inorder to obtain increased gages of finished product.

It is another object of the invention to produce uniform fine austenitegrain size before pancaking and apply less pancaking to produce thickergage product, which exhibits homogeneous properties without anisotropydue to unfavorable crystallographic texture development.

It is yet another object of the invention to obtain consistently lowductile to brittle transition temperature (DBTT) and good drop weighttear test (DWTT) performance. DWTT properties are empirically correlatedwith thickness of pancaked austenite grain. By refining the austenitegrain size, less pancaking is required to meet target DWTT properties.

SUMMARY OF THE INVENTION

It has now been discovered that the addition of niobium totitanium-bearing super-martensitic stainless steel refines the austenitegrain size due to the formation of titanium-niobium bearing compositeprecipitates. This led to the present invention's development ofnano-scale precipitation engineering of TiN—NbC composite precipitatesto prevent austenite grains from coarsening. According to the presentinvention, TiN precipitates, which are formed just after solidificationin the continuous cast slab, are used to control the inter-particlespacing, while NbC precipitates growing on pre-existing TiN particlesare used to control the size of the precipitates, both size and spacingof TiN—NbC composite precipitates are the key to pinning austenitegrains of the required size to prevent them from coarsening. The drivingforce for grain coarsening is capillary force, which can be determinedfrom the equation: capillary force=2 γ/R, where R is the radius ofcurvature of the grain boundary and y is the surface energy of theboundary.

The driving force for grain coarsening tends to decrease as the grainsize increases. In accordance with the present invention, a 30 microngrain size is targeted instead of the conventional 60 microns. Thisdriving force for boundary movement is counteracted if particles pin theboundary. The pinning force increases with the number density and sizeof the particles. Thus, the driving force for grain coarsening when thetarget grain size is 30 microns can be determined from the numberdensity of particles [TiN], which sets up the interparticle spacing thatcan be measured, e.g., 200 nm. But the particle size of TiN is toosmall, about 15 nm. The limiting austenite grain size is 90 microns,which is rather coarse. By growing NbC, TiN—NbC composites can beformed, which are now large, about 25 nm. The limiting austenite grainsize is 32 microns, which is close to target. By growing to 30 nm, ascan be seen from Table 1 below, the limiting austenite grain size isdecreased to 22 microns. It should be noted that the number density andthe volume fraction of precipitates are controlled by the thermodynamicsand kinetics of precipitation which, in turn, depend upon the chemicalcomposition and processing parameters of the steel.

TABLE 1 Zener limiting Austenite grain size in micrometers Increasing Nconcentration Increasing base Nb concentration Inter Particle ParticleDiameter, nm Distance, nm 10 15 20 25 30 35 40 50 60 70 80 150 85 38 2114 10 7 5 3 2 1.8 1.3 200 203 90 51 32 22 17 13 8 6 4 3 250 397 176 9963 44 32 25 16 11 8 6 300 687 305 171 110 76 56 43 28 19 14 11 350 1091485 273 174 121 89 68 44 30 22 17 400 1629 724 407 260 181 133 102 65 4533 25 450 2320 1030 580 371 257 190 145 93 65 47 36 500 3183 1414 795509 353 259 199 127 88 65 50 550 4236 1882 1059 678 471 346 265 170 11886 66

The validity of the mechanism underpinning the technology of nano-scaleprecipitation engineering for austenite grain size control in upstreamprocessing is demonstrated by experimental results on line pipe gradesprocessed under plate rolling and hot strip rolling conditions. Thepresent invention provides a platform for austenite grain size controlin upstream processing of austenite of niobium microalloyed steels towhich downstream processing and final properties of the product arerelated.

A process for controlling austenite grain size in austenite processingthrough nano-scale precipitation engineering of TiN—NbC composites toproduce thicker gage product of niobium microalloyed steel comprisescontrolling the base chemical composition of a steel product to includeabout 0.003-0.004 wt % nitrogen, 0.012-0.015 wt % titanium, 0.03-0.07 wt% carbon, and 0.07-0.15 wt % nobium; lowering the temperature ofroughening to end the roughening operation in the temperature range offrom about 980° C. to 1030° C.; retaining greater than about 0.03%niobium in solution in the matrix by rapid cooling of the product toenter the finish rolling operation below the temperature of norecrystallization, with an austenite grain size of about 30 microns; andapplying reduced rolling reduction in the finish rolling operation.Lowering the temperature of roughening prevents grain refined austenitefrom coarsening above about 30 microns by formation of TiN—NbC compositeprecipitates. Applying reduced rolling reduction in the finish rollingoperation acts to pancake the fine austenite grain size of about 30microns to obtain a sufficient surface to volume ratio to producethicker gage resulting steel product.

The grain size can be controlled in the range of about 20-40 microns atentry to the finish rolling operation. TiN precipitates can be in therange of about 10-20 nm and the inter-particle spacing can be about200-300 nm. Thermodynamic potential for precipitation of NbC can occurtowards the end of the roughing operation at temperatures ranging fromabout 980° C. to about 1030° C. TiN—NbC composites can be in the sizerange of about 20-50 nm. The process can include applying acceleratedcooling upstream between the end of the roughing operation and the startof finish rolling to avoid depletion of solute niobium from the matrixto less than about 0.03 wt percent. Accelerated cooling of the productcan be applied to avoid rolling in the partial recrystallization regime.The process can include controlling nitrogen at or below about 40 ppmand making a titanium addition to meet the stoichiometric requirement tocombine with all nitrogen to form high number density of TiN precipitatein about the 10-20 nm size range. The process can include processing thesteel product by conventional plate rolling, conventional hot striprolling, steckel mill rolling, and/or near net shape processing. Thesteel product can be line pipe steel, infra-structure steel, and/orsupermartensitic stainless steel. The crystallographic texture-relatedanisotropic properties of the resulting steel product can be minimized.The process can include substituting titanium partially or fully in thebase chemistry with a member of the group consisting of Zr, Hf, Ta, W,V, Cr, Mo, Al and mixtures thereof, each with high affinity for nitrogento form nano-scale precipitates on which NbC can grow epitaxially togive composite precipitates.

The process also can include partially substituting niobium in the basechemistry with other microalloying elements with high affinity forcarbon selected from the group consisting of Zr, Hf, Ta, W, V, Cr, Moand mixtures thereof, each to give composite precipitates. The processalso can include substituting solute niobium on entry to finish rollingwith other elements, which exhibit solute drag comparable to niobium.Still further, the process can include rapidly cooling the steel productto enter finish rolling at a temperature at or below about 920° C. Therolling reduction can be reduced substantially by more than about 15%.The steel product can exhibit a gage thickness of about 17-30 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show Electron Back-Scattered Diffraction (EBSD) Images,revealing austenite grain size of specimens of two 13% Cr-5% Ni-2% Mosupermartensitic stainless steels. FIG. 1A corresponds to the controlsteel without niobium additions, and FIG. 1B is the sample with 0.1 wtpercent niobium addition, both steels contain titanium, as shown inTable 2. The addition of 0.1 wt percent niobium decreased the austenitegrain size from 80 to 35 microns. i.e., titanium by itself could onlyproduce an austenite grain size of 80 microns. But it is only with theaddition of 0.1 wt. percent niobium, that austenite grain size can bedecreased from 80 to 35 microns under identical processing conditions.

FIG. 2 is a TEM image of precipitates extracted on a carbon replica,showing nano-scale Ti—Nb bearing precipitates in the size range of 25-30nm with a mean interparticle spacing of 230 nm. Energy dispersiveanalysis of the precipitates is shown alongside. The spectrum showsX-ray signals characteristic of titanium and niobium. The precipitatesappear to be TiN—NbC composites similar to those found in line pipesteels, see FIGS. 4, 12 and 13.

FIG. 3 is a plot of driving force for grain coarsening of austenite as afunction of austenite grain size. The coarsening is driven by reductionin surface energy of the grains. This is counteracted by particlepinning the boundary. The pinning pressure is governed by the particlesize and number density. The number density determines interparticlespacing. Thus, small interparticle spacing and increased particle sizeare required to increase pinning pressure to counteract and preventgrain coarsening of fine grains. The particle limited grain size withand without niobium shows the effectiveness of TiN—NbC compositescompared with TiN precipitates in pinning fine grains.

FIG. 4 is a high resolution TEM image of TiN—NbC composites obtained inlow nitrogen line pipe steel microalloyed with titanium and niobium.Energy dispersive analysis shows X-ray signals characteristic of niobiumand titanium in the composite precipitates. NbC precipitates appear tohave grown on preexisting cuboidal TiN precipitates.

FIGS. 5A and 5B are salient results from my previous work on themicrostructural evolution of TiN—NbC composites in low interstitialtitanium-niobium microalloyed steels investigated by hot torsionsimulation of rolling. Based on quantitative analysis of thermodynamicpotential for precipitation, mole fraction of TiN—NbC is plotted as afunction of temperature. FIG. 5A shows the precipitate evolution curvefor the high niobium low interstitial steel-G, containing carbon 0.03,nitrogen 0.003, titanium 0.014 and niobium 0.095 wt percent.Thermodynamic potential for precipitation of NbC starts at 1060° C. FIG.5B shows the mean flow stress from hot torsion simulation (shown as opencircles) as a function of the inverse of the absolute pass temperaturefor Steel-G. The bold line is the flow stress pertaining to a fullyrecrystallised steel. The onset of recrystallization retardation startsat 1060° C. corresponding to the onset of the thermodynamic potentialfor precipitation of NbC. Growth of NbC on preexisting precipitates ofTiN is confirmed in this work, which obviates the need for independentnucleation of NbC. Thus, the resulting TiN—NbC composite retardsrecrystallization, causing the increase in flow stress detected by hottorsion rolling simulation results.

FIG. 6 is a schematic diagram that inter-relates the increase in size ofTiN—NbC composite to volume fraction of NbC, which is determined by thethermodynamic potential for precipitation of NbC. The interparticlespacing is fixed by TiN on which NbC grows. This diagram illustratesthat the rough rolling temperature window has to be lowered so thatthermodynamic potential for growth of NbC is obtained on pre-existingTiN precipitates to form TiN—NbC composites at the end of rough rolling.

FIG. 7 is a process flow diagram of prior technology in which there isno intentional control of austenite grain size in upstream processing ofrough rolling, and the austenite grain size on entry to finish rollingmay be coarse, generally ranging in size from 60-80 microns. Therefore,heavy rolling reduction is applied in finish rolling stands downstreamto reduce the thickness of pancaked austenite in order to obtain goodtoughness at low temperature in the final product. This limits thethickness of the final product generally well below 16 mm, processed byconventional plate rolling or conventional hot strip rolling of niobiummicroalloyed steel. This is illustrated with the specific example ofSteel-A of 10 mm gage, with a high nitrogen content of 75 ppm. Roughrolling is carried out in the temperature window above the equilibriumtemperature for precipitation of NbC. TEM characterization shows coarseprecipitate of mean size 83 nm with a large interparticle spacing of 550nm, which gives a Zener limiting austenite grain size of 62 microns.This requires heavy rolling reduction for pancaking austenite grains,resulting in thinner gage (<16 mm).

FIG. 8 is a process flow diagram based on the present invention whereinaustenite grain size upstream is controlled by the size and spacing ofTiN—NbC composite precipitates, which is referred to herein as“nano-scale precipitation engineering.” The austenite grain size isintentionally controlled to be fine with a target grain size under 30microns. This requires less rolling reduction to reduce the thickness ofpancaked austenite grain size in order to obtain good toughness at lowtemperature as measured by percentage shear area in DWTT tests. As aresult of applying less rolling reduction to the transfer bar, thethickness of the final product processed by conventional plate rollingor conventional hot strip rolling of niobium microalloyed steel can beincreased well above 16 mm. This is demonstrated with the specificexample of Steel-C. The steel contains a low nitrogen content of 0.004wt percent and stoichiometric addition of Ti to combine with nitrogen.TEM characterization shows high number density of TiN precipitates withan interspacing of 220 nm. The end of rough rolling temperature islowered to a temperature in the range from 980 to 1030° C., preferably1000° C. to promote growth of NbC on pre-existing TiN to give TiN—NbCcomposites of 32 nm size. Electron energy loss spectroscopy hasconfirmed growth of NbC on pre-existing TiN. The limiting austenitegrain size by TiN—NbC composite precipitates is less than 30 microns,which requires less pancaking in finish rolling, resulting in thickergage (>16 mm).

FIG. 9 is a montage that relates interparticle spacing of nano-scaleTiN—NbC composites to titanium and nitrogen content in the base chemicalcomposition, which is mapped on the equilibrium solubility product forTiN precipitation as a function of temperature. The montage represents acomprehensive data base on inter-particle spacing of TiN obtained inline pipe steel in which nitrogen content is varied. The interparticlespacing of TiN is in the 200-250 nm range when nitrogen content islowered to 40 ppm, titanium is added in the stoichiometric requirementto combine with all the nitrogen. High number density of TiN—NbCcomposite precipitates occurs in the size range of 25-35 nm. Bycontrast, when nitrogen content is raised to 75 ppm, the interparticlesize is large at about 550 nm, and the particle size is coarse (80 nmsize).

FIG. 10 is an optical micrograph showing austenite grain size in thetransfer bar of Steel-D quenched after shearing. The austenite grainsize is about 48-55 microns. This is in agreement with Zener limitingaustenite grain size, based on measured values of precipitate size andinterparticle spacing of Steel-D, shown in Figure-9.

FIG. 11 is Kozazu's diagram, inter-relating rolling reduction andaustenite grain size with surface to volume ratio, Sv factor, ofpancaked austenite grain size, to which the final structure andproperties can be related. Kozazu's diagram shows that a large rollingreduction (70 percent) is required to pancake coarser austenite grain of70 micron compared with lower rolling reduction (<50 percent) requiredto pancake finer austenite grain of 30 micron grain size to achieve thesame surface to volume ratio, i.e., Sv factor.

FIG. 12 is a photograph compiled from Electron Energy Loss Spectroscopy(EELS) data of nano-scale TiN—NbC precipitates observed in nano-scaleprecipitation engineered high grade line pipe steel processed byconventional hot strip rolling. The epitaxial growth of NbC on faces ofthe TiN cubic precipitates can be clearly seen in Steel-C(X-90 grade).

FIG. 13 shows elemental mapping from EELS data of the TiN—NbC compositeprecipitates, shown in FIG. 12. These results show unambiguouslyepitaxial growth of NbC on pre-existing TiN.

FIG. 14 illustrates the application of nano-scale precipitationengineering of TiN—NbC composite precipitates for austenite grain sizecontrol in near net shape processing for a typical lay out of milldesign with three roughing stands.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A and 1B are electron backscatter diffraction images of 13% Cr-5%Ni-2% Mo super-martensitic stainless steels without and with 0.1 wtpercent niobium addition. These steel specimens were identicallyprocessed, solution treated at 1050° C. and air-cooled. The detailedchemical compositions of the two steels are given in Table 2.

TABLE 2 Chemical composition of two 13% Cr—5% Ni—2% Mo steels withoutand with niobium in wt percent Steel grade C Si Mn P S Cr Ni Mo N Nb Ti13Cr5Ni2Mo 0.020 0.42 0.51 0.016 0.004 12.59 5.01 1.90 0.013 — 0.006213Cr5Ni2MoNb 0.022 0.41 0.48 0.016 0.006 12.91 5.16 2.05 0.010 0.110.0043

The steel without niobium addition but with titanium exhibits anaustenite grain size of 80 microns, which shows that titanium additionalone is not effective in refining austenite grain size. But with theaddition of 0.1 wt percent niobium, the austenite grain size issignificantly decreased to 35 microns. The white lines in FIG. 1delineate the austenite grain boundaries.

FIG. 2 shows TEM images of composite precipitates of TiN—NbC observed inthe steel with a niobium addition. The mean inter-particle spacing ofcomposite precipitates is 231 nm and the mean particle size is increasedfrom 15 nm for TiN to 30 nm for the TiN—NbC composite particles. Thepinning pressure exerted by the particles of TiN—NbC is 0.08 MPa, whichcounteracts the driving force for grain coarsening of austenite of grainsize 35 microns as shown in Table 3.

TABLE 3 Calculated limiting austenite grain size by TiN and TiN—NbC in13Cr5Ni2Mo steel with 0.1 wt. percent niobium Zener Limiting pinningaustenite Size Inter-particle Volume pressures grain size Precipitate(nm) Spacing (nm) fraction (MPa) (μm) TiN 15 231 0.00015 0.02088 134TiN—NbC 30 231 0.00123 0.08028 35

The area occupied by particles on the boundary must be recreated beforethe boundary moves, and this is the energy preventing grain coarseningand is referred to as Zener drag. Solute atoms piling up at theinterface exert a drag force on boundary mobility, which is referred toas solute drag. Solute drag of niobium is less pronounced in the hightemperature window. Zener drag is increased as the interparticledistance is reduced and the particle size becomes bigger for a givenvolume fraction of precipitate, which is determined by the thermodynamicpotential for precipitation for a given steel composition. TiN andTiN—NbC precipitates occur on a nano-scale and therefore engineering thesize and dispersion of nano-scale precipitates is termed “engineeringnano-scale precipitates for pinning grain boundaries.”

FIG. 3 shows that TiN particles by themselves are not effective inpinning austenite grains. The epitaxial growth of NbC on pre-existingTiN effectively increases the particle size from 15 to 30 nm, with thecorresponding increase in Zener pinning pressure nearly threefold (2.5times) compared with TiN alone and thus decreases the limiting austenitegrain size. This composite precipitate involves growth of NbC onpre-existing TiN, which is to be distinguished from MX type precipitatesreported in the literature and previous U.S. Pat. No. 6,899,773.

FIG. 4 and FIG. 5 provide a summary of the prior work ontitanium-niobium microalloying, which shows that if nitrogen iscontrolled under 40 ppm and titanium additions are made to thestoichiometric requirement of N to form TiN, a high number density ofTiN can be promoted. On reaching the temperature where thermodynamicpotential for precipitation of NbC occurs, NbC will start to grow onpre-existing TiN. This work was reported by S. V. Subramanian, F.Boratto, J. J. Jonas and C. M. Sellars and published in the Proceedingsof International Symposium on “Microalloyed Bar and Forging Steels”edited by Mike Finn, CIM, held at Hamilton, Ontario, Canada, Aug. 26-29,1990, pp. 120-136.

FIG. 4 shows a TEM image of TiN—NbC composite precipitates. The energydispersive analysis and EELS (Electron energy loss spectrum) haveconfirmed that NbC precipitates grow epitaxially on the faces ofcuboidal precipitates of TiN (with the NaCl crystal structure.)

FIG. 5 shows that growth of NbC on pre-existing TiN can be detected byan increase in flow stress during hot torsion simulation of rolling.This temperature is found to coincide with the equilibrium temperatureof precipitation of NbC, as shown in Table 4. The implication is thatthe volume fraction of NbC growing on pre-existing TiN can be determinedby the thermodynamic potential for precipitation of NbC.

TABLE 4 Effect of chemical composition on precipitation kinetics ofTi—Nb microalloyed steel during hot torsion simulation of rolling;Growth of NbC on pre-existing TiN occurs close to the equilibriumtemperature for precipitation of NbC, which can be detected by flowstress increase during hot torsion. T_(NbC) Sellars Model T_(nr) BaseChemistry Equilibrium RLT RST Hot Torsion Steel C N Ti Nb ° C. ° C. ° C.° C. Fe—Nb—C 0.15 0.005 0 0.031 1130 955 922 960 Line pipe A 0.0290.0023 0.013 0.055 1010 898 880 1010 Line pipe B 0.026 0.0027 0.0200.057 1020 900 885 1016 Line pipe C 0.016 0.0018 0.013 0.049 960 850 836994 Line pipe G 0.027 0.0035 0.014 0.099 1060 936 923 1060 Line pipe K0.027 0.0019 0.014 0.093 1050 936 923 1040 RLT = Recrystallization LimitTemperature RST = Recrystallization Stop Temperature T_(nr) =Temperature of No Recrystallization

The breakthrough in austenite grain size control upstream arose out ofobservations in super-martensitic stainless steel, where a titaniumaddition by itself did not produce a fine austenite grain size. But whencombined with niobium additions, a fine austenite grain size wasobtained. This was caused by TiN—NbC composite precipitates, formed byNbC growing on pre-existing TiN. The number density of precipitates wascontrolled by the TiN. By increasing the size of the precipitates bygrowth of NbC on pre-existing TiN, it is possible to increase thepinning pressure of precipitates to arrest austenite grain boundarymovement at the required austenite grain size. This discovery underliesthe present invention of nano-scale precipitation engineering. Theaustenite grain size is controlled by inter-particle spacing of TiNprecipitates and the size of the precipitates, each of which can beindependently controlled by design of the base steel composition. A highnumber density of TiN is promoted when the precipitation occurs in thematrix at low temperature, which calls for lowering the nitrogen contentand adding titanium to the stoichiometric requirement to form TiN,providing one atom of titanium for every one atom of nitrogen. Bylowering nitrogen to less than 40 ppm and adding titanium to thestoichiometric requirement of about 0.014 wt percent, the inter-particlespacing was found to be around 200 nm and the TiN precipitate size wasfound to be in the 10-15 nm range. The Zener pinning pressure on theboundary is relatively small, capable of arresting austenite grains ofabout 80 microns from coarsening. The pinning pressure can be increasedby growing NbC precipitates on TiN, thereby increasing the size of thecomposite precipitates of TiN—NbC. This requires lowering thetemperature window of roughing so that the thermodynamic potential forgrowth of NbC on pre-existing TiN is obtained to form TiN—NbCcomposites. By increasing the particle size while retaining the sameinter-particle spacing, the pinning pressure is increased to arrest thefiner austenite grains from coarsening. By increasing the size ofprecipitates to 30 nm with an interparticle spacing of 220 nm, the Zenerpinning pressure is increased to prevent austenite grain size of 30microns from coarsening.

The concept of nano-scale precipitation engineering to arrest graincoarsening is illustrated in the schematic diagram given in FIG. 6. Theupstream processing of austenite for austenite grain size controlrequires a high number density of precipitates with short interparticlespacing and adequate precipitate size with good dispersion to applyadequate pinning pressure to prevent coarsening of fine grains ofaustenite obtained at the end of roughing. This innovation relates toproduct-process integration, where refinement of austenite grain size inupstream processing by Zener pinning by TiN—NbC composite precipitatesof grain refined austenite to prevent grain coarsening is used to reducetotal rolling reduction in finish rolling downstream to produce thickergage product.

Nitrogen is controlled to promote the formation of TiN precipitates atlower temperatures. The resulting finely-dispersed nano-precipitates ofTiN then act as scaffolds for the epitaxial formation of NbC, therebyraising the volume fraction of dispersed composite precipitates by afactor of about 3X. This is sufficient to hold the austenite grain sizeto about 30 micrometer size in low nitrogen steel compared to 60 micronsfor higher nitrogen steel. The advantage here lies in the reducedaustenite grain size, permitting the application of a reduced rollingreduction during final processing and the consequent ability to producethicker gages of higher strength material (X-70, X-80, X-90, X-100)compared with high nitrogen steel, which requires heavy rollingreduction that limits final gage of the product.

FIG. 7 is the flow diagram of product-process integration of the priorart technology without any intentional control of austenite grain sizeupstream and its consequence on heavy packing downstream resulting inthin gage product. A high nitrogen content in the base compositionresults in coarse precipitates of TiN with a large interparticle spacingof 550 nm. TEM characterization shows the coarse precipitate of TiN.Rough rolling is carried out in a temperature window above theequilibrium temperature for precipitation of NbC. Thus, Zener limitingaustenite grain size is 62 microns, as shown in the Table in FIG. 7.Therefore, heavy pancaking is required to obtain consistently good DWTTperformance.

By comparison, FIG. 8 is the flow diagram of product-process integrationof the present invention based on austenite grain size control byengineering size and spacing of TiN—NbC composite precipitates and itsconsequence on reduced rolling reduction downstream, resulting inproduction of thicker gage product. A low nitrogen content of 40 ppmwith stoichiometric addition of Ti to combine with all nitrogen promotesin high number density of TiN precipitates with an interparticle spacingof 220 nm. The temperature window of roughing is lowered below theequilibrium temperature for precipitation of NbC to promote growth ofNbC on pre-existing TiN, which is confirmed by EELS characterization ofTiN—NbC composite precipitates. The limiting austenite grain size isbelow 30 microns. Therefore, less rolling reduction is applied toproduce thicker gage product.

The technology of nano-scale precipitation engineering of TiN—NbCcomposites involves two microstructural parameters. The first is theinterparticle spacing. The second is the particle size. This inventionis based on the discovery that TiN—NbC composites offer a window ofopportunity to control interparticle spacing through optimum TiNdistribution and the size of the particle by epitaxial growth of NbC onpre-existing TiN particles. The first step is to engineer a high numberdensity and uniform dispersion of TiN particles. This is done bypromoting nucleation of TiN in austenite at lower temperatures throughcontrol of the base steel chemical composition. Since the precipitatesoccur on a nano-scale, it is essential to characterize the precipitatesby transmission electron microscope. The well-known carbon replicatechnique is used in this work to extract the precipitates occurring inbenchmarked steels. FIG. 9 shows a comprehensive database of four benchmarked steels in which nitrogen content is varied under different millprocessing conditions. The chemical compositions of the four steels aregiven in Table 5.

TABLE 5 Effect of varying nitrogen content on thermodynamic potentialfor precipitation of TiN, and its consequence on ppt size and Zenerpinning pressure, Zener limiting austenite grain size TiN Inter- TiN—NbCVolume fraction N Ti C Nb particle Ppt size of TiN—NbC Zener pinningLimiting austenite I.D. (wt %) (wt %) (wt %) (wt %) spacing in (nm) (nm)at 1000° C. Pressure (MPa) grain size (μm) A 0.0075 0.015 0.06 0.09 55383 0.00176 0.044 62 B 0.0035 0.014 0.07 0.08 218 32 0.0016 0.108 26 C0.0040 0.015 0.05 0.09 221 32 0.00159 0.104 27 D 0.0055 0.012 0.0480.067 397 52 0.00117 0.047 59

Steel-A with the highest nitrogen content of 0.0075 wt. percent exhibitsa large mean inter-particle spacing of about 550 nm compared withSteels-B and C with a low N content of 35-40 ppm, which exhibit a meaninter-particle spacing of about 220 nm. Steel-D with intermediatenitrogen content of 55 ppm exhibits an intermediate interparticlespacing of about 400 nm. Clearly, the inter-particle spacing of 220 nmcan be achieved by lowering nitrogen content to or below 40 ppm andadding titanium to the stoichiometric requirement to tie up all thenitrogen. The precipitate size of TiN—NbC of the highest nitrogenSteel-A is 83 nm, which gives Zener limiting austenite size of 62microns. By comparison, Steel-B and Steel-C with low nitrogen give Zenerlimiting austenite grain size of about 27 microns. FIG. 10 shows theaustenite grain size measured in the center of a thick transfer bar of53 mm of Steel-D, quenched after rough rolling with an intermediatenitrogen content of 55 ppm and an inter-particle spacing of 397 nm. Thepredicted Zener limiting austenite grain size is 59 microns, whichcompares well with the measured value of 55 microns, which validates theapproach. Thus, nano-scale precipitation engineering offers a soundmetallurgical basis for controlling austenite grain size during upstreamprocessing of austenite.

In conventional processing of conventional nitrogen-bearing niobiummicroalloyed steel (0.005-0.008 wt. percent nitrogen), roughing iscarried out where there is no thermodynamic potential for precipitationof NbC. The loss of niobium by excessive growth of NbC on pre-existingTiN particles is reduced by minimizing the time of processing in themill. In the case of nano-scale precipitation engineering of TiN—NbCcomposites with finer inter-particle spacing, it is even more criticalto prevent depletion of solute niobium in the matrix by acceleratedcooling upstream between the end of roughing and the start of finishrolling. It is essential to control the finish rolling entry temperaturebelow the temperature of no recrystallization in order to avoid rollingin the partial recrystallization regime, which requires acceleratedcooling. Thus, accelerated cooling is required to prevent depletion ofsolute niobium by precipitate growth, subsequent to pinning theaustenite grains of the required size in higher grade line pipe steel.

TABLE 6 Effect of austenite grain size (GS) and percent reduction belowtemperature of no recrystallization (T_(NR)) on Sv factor and ferritegrain size. Sv Factor Austenite % Reduction Ferrite GS (um) mm²/mm³ GS(um) below T_(NR) 9 80 40 60 9 80 30 30 9.4 70 55 60 9.4 70 35 30 11 6070 60 11 60 40 30

Table 6 is extracted from Kozazu's diagram in FIG. 11, which illustratesthe benefit of austenite grain refinement before pancaking in reducingthe rolling reduction below the temperature of no recrystallization toachieve the same surface to volume ratio. Thus, by reducing theaustenite grain size from 40 to 30 microns, the rolling reduction can bedecreased from 60 to 30 percent to attain the same Sv factor of 80mm²/mm³ in order to obtain ferrite grain size of 9 micrometers andconsequently the gage (thickness of final product) can be significantlyincreased. It is well established that by refining the austenite grainsize upstream, excellent strength and fracture properties can beobtained in thicker gage product. Wenjin Nie et al. have demonstratedthe importance of austenite grain size control on final DWTT propertiesof heavy thick X-80 pipe line steels (Advanced Materials Research, Vols.194-196, (2011), pp. 1183-1191).

EXAMPLES

The principal differences in the processing of higher niobium steelsbetween the prior technology without austenite grain size controlupstream and the technology of the present invention based on austenitegrain size control are examined in further detail and their consequenceon product in terms of gage thickness and properties are highlighted inthe following examples.

Example 1 Plate Rolling Steel-A with High Nitrogen Content

Steel-A is representative of prior technological practice, where ahigher nitrogen content of 75 ppm gives coarse TiN particles with largeinter-particle spacing of 550 nm. Roughing is carried out in thetemperature window where there is no thermodynamic potential forprecipitation of niobium carbide. Thus, the austenite grain sizeentering finish rolling is 60-80 microns. This then requires heavypancaking below the temperature of no recrystallization. Thus, the finalgage is generally limited to 16 mm. Typical property results obtainedfrom 10 mm gage are reproduced below in Table 7:

TABLE 7 End of Limiting austenite N Ti C Nb roughing grain size by TiN0.0075 0.015 0.06 0.088 1100° C. 90 microns DWTT % SA at −7° C.: 100%;CVN toughness at −7° C.: 140 Joules; Yield Strength/Rp0.5: 610 MPa;Ultimate Tensile Strength/Rm: 714 MPa

Example 2 Steel-E: Plate Rolling with Low Nitrogen Content

Steel-E has a lower nitrogen content (40 ppm) with titanium and niobiumaddition comparable to Steel-A. The low nitrogen and stoichiometricaddition of titanium to combine with nitrogen to form TiN has produced ahigh number density of TiN with a mean interparticle spacing of 220 nm.This steel was processed under two distinctly different conditions. Thefirst set of conditions was where the rough rolling window was similarto Steel-A, that is where there is no thermodynamic potential for NbCprecipitation to occur. Under these conditions, TiN particles alone arenot able to develop pinning pressure adequate to pin a fine austenitegrain size. Thus, the resulting coarse austenite grain size warrantsheavy rolling reduction, which is not possible to achieve in 22 mm gagethickness. As a consequence, the final product fails as percentage sheararea in the DWTT specimen is lowered to 55 percent at −15° C. (See Table8).

Steel-E was also processed under a second set of conditions, wherethermodynamic potential occurs for growth of NbC on pre-existingparticles at the end of rough rolling. In this case, NbC grows onpre-existing TiN to increase the particle size so that the pinningpressure is increased to prevent austenite grain coarsening above 30microns.

Once austenite grain size is refined at the entry to finish rolling,less rolling reduction is required in finish rolling in accordance withKozazu's diagram in FIG. 11 to obtain adequate surface to volume ratioto obtain fine grains in the final product. In this case, 100 percentshear area on the fracture surface in DWTT is obtained. This exampleshows that TiN by itself cannot grain refine austenite even though theinterparticle spacing may be fine unless the particle size is increasedby growth of NbC on pre-existing TiN particle. This example demonstratesthe importance of lowering the temperature window of roughing to promotegrowth of NbC on pre-existing TiN to limit austenite grain coarsening atthe end of roughing in order to produce 22 mm gage with excellent DWTTperformance.

Steel-E

22 mm thick gage —X80 Plate: (nitrogen 0.004, titanium 0.016, carbon0.05, niobium 0.1)

Effect of processing temperature window on low nitrogen and high niobiumsteel.

Effect of rough rolling in the temperature window with and withoutthermodynamic potential for precipitation of NbC.

TABLE 8 #2 Condition #1 Condition (Roughing (Roughing to without NbCgrowth on promote NbC Heat TiN) growth on TiN) A_(k) −20° C./Joules 328372 DWTT (−15° C.) average 55 98 SA %

Example 3 Conventional Hot Strip Rolling Steel-D with IntermediateNitrogen Content

Steel-D presents a case, where nitrogen content is at an intermediatelevel of about 55 ppm and therefore the mean inter-particle distance is390 nm. Though the temperature of finish rolling promoted growth of NbCon pre-existing TiN, the TiN—NbC composite did not have adequate pinningpressure to arrest austenite grains finer than 59 microns, see Table 9.This is partly due to low niobium content, i.e. 0.067 wt. percent. Thus,the strip rolled to 20 mm gage thickness exhibited 100 percent shearonly at −10° C. and above, see Table 10a and 10b.

Steel-D: 20 mm thick gage X80 Strip

High nitrogen and lower niobium with rough rolling in the temperatureregime where there is thermodynamic potential for precipitation of NbC.

TABLE 9 Limiting Pancaking End of austenite Pancaking austenite N Ti CNb roughing grain size reduction grain 0.0055 0.012 0.048 0.067 980° C.59 microns 62.6 20 microns

TABLE 10a Ultimate tensile Yield strength/Rp0.2 Strength/Rm Yield ratioTotal elongation/% 588 MPa 670 MPa 0.88 48

TABLE 10b Test DWTT Shear Area % Temperature/ Charpy V-notched T L 45° °C. toughness/Joules direction direction direction 0 — 95 95 95 −10 — —100 90 −20 485 70 95 100

Example 4 Conventional Hot Strip Rolling

In-depth characterization of Steel-C has confirmed that theinterparticle spacing of TiN is 220 nm. Steel-C represents low nitrogencontent, with optimized addition of titanium to promote high numberdensity and uniform dispersion of TiN with an inter-particle spacing of220 nm. The end of roughing is in the temperature window wherethermodynamic potential for precipitation of NbC occurs.

TEM-EELS characterization of TiN—NbC precipitates shown in FIGS. 12 and13 confirms epitaxial growth of NbC on pre-existing TiN particles. Thissteel exhibits remarkable toughness at very low temperature (−40° C.),see Tables 11a and 11b. The steel exhibits uniformity of microstructurewhich is less prone to anisotropic properties due to unfavorable texturedevelopment.

Steel-C: 16.4 mm thick gage X90 Strip.

Low nitrogen and higher niobium with rough rolling in the temperatureregime where there is thermodynamic potential for precipitation of NbC.

TABLE 11a Yield strength/ Ultimate tensile Total Uniform Rp0.2Strength/Rm Yield ratio elongation/% elongation/% 670 MPa 800 MPa 0.8417 5.6

TABLE 11b DWTT Testing Temperature/ Charpy V-notched (T direction) ° C.toughness/Joules Shear area % 10 313 0 302 100 −10 300 100 −20 315 100−40 318 100 −60 329

Example 5 Compact Strip Processing and Thin Slab Processing

There are different mill designs available for compact strip processing.Nano-scale precipitate engineering of TiN—NbC composites offers ageneric platform for preventing austenite grain coarsening bycontrolling interparticle distance by TiN, and particle size by NbCgrowing on the pre-existing TiN. In near net shape processing, in somecases, the transfer bar is reheated for the purpose of temperaturehomogenization, then the austenite grains inevitably coarsen in theabsence of second phase particles. The technology of nano-scaleprecipitation engineering offers a sound basis for pinning austenitegrain boundary with TiN—NbC composite precipitates at the end ofroughing, and also during reheating. This process can be combined withaccelerated cooling to prevent depletion of solute niobium by excessivegrowth of NbC, over and above the composite particle size required toprevent grain coarsening of austenite of a specific grain size. Trialsof nano-scale precipitation engineering in a mill with two roughingstands and accelerated cooling at 4° C./s have given uniformity ofmicrostructure, which is beneficial in achieving consistent strength andfracture properties.

The application of nano-scale TiN—NbC composite precipitationengineering offers a generic platform for austenite grain size controlin upstream processing. A potential application to in-line strip rollinginvolving three roughing stands to produce X-80 grade strip of 15 mmgage is illustrated in FIG. 14 along with critical processingparameters.

The foregoing examples distinguish the principal differences inprocessing higher niobium steels between the prior art withoutintentional austenite grain size control upstream and the presentinvention based on austenite grain size control and the consequencesthereof on product in terms of gage thickness and properties. Thesalient points are summarized in Table 12.

TABLE 12 ID Prior Art Present Invention 1 No specific nitrogen target Ncontrol (nitrogen 0.003-0.004, titanium 0.012-0.015) 2 Coarse andnon-uniformly dispersed TiN Fine uniformly dispersed and high numberdensity of TiN 3 Roughing in temperature range where there is Roughingin temperature range where there is no thermodynamic potential forprecipitation of thermodynamic potential for precipitation of NbC NbC 4NbC growth on coarse TiN precipitates before High number density of TiNand adequate entry to finish rolling; Inadequate Zener drag to volumefraction of TiN—NbC composite prevent grain coarsening of fine austeniteprecipitates nano scale engineered to give grains adequate Zener drag topin fine austenite grains from coarsening 5 Fast cooling betweenroughing and finish Fast cooling (laminar cooling at 4° C./s) betweenrolling is beneficial for retaining niobium in roughing and finishrolling is essential to retain solution adequate niobium in solution 6Coarsened austenite grains 50-70 μm at entry to Zener limiting austenitegrain size (30 μm) at finish rolling entry to finish rolling 7 Heavypancaking is required (total reduction Less pancaking (total reduction50-66%) is 66-80%) to achieve target Sv factor for coarse adequate toachieve target Sv factor for austenite austenite grain size in 50-70 μmgrain size under 30 μm 8 Production limited to thinner gage Productionof thicker gage high grade product product (10-17 mm); potential for(17-30 mm) unfavorable texture development less texture relatedanisotropy

Process Steps for Controlling Austenite Grain Size in UpstreamProcessing of Austenite:

According to the present invention, the process steps for controllingaustenite grain size upstream before entry to finish rolling to producethicker gage product are given below:

(i) Lower the nitrogen content in the base chemistry to 30-40 ppm andadd titanium to the stoichiometric requirement (0.012-0.015 wt percenttitanium) to combine with all nitrogen to form in austenite high numberdensity of TiN precipitates in the size range of 10-20 nm with aninterparticle spacing of 200-300 nm, before the start of roughing;

(ii) Refine austenite grain size by static recrystallization in roughrolling to a target grain size of 10-30 microns but preferably 10-20microns at the end of roughing;

(iii) Adjust carbon content in the range of about 0.03 to 0.07 wtpercent but preferably 0.04-0.05 wt percent and niobium in the range ofabout 0.07 to 0.15 but preferably 0.09 to 0.1 wt percent so thatthermodynamic potential for growth of NbC on pre-existing TiN to formTiN—NbC composites occurs towards the end of roughing, i.e., between980°-1030° C.;

(iv) Target TiN—NbC composites to grow to 25-50 nm but preferably 25-30nm so that pinning pressure from TiN—NbC composites of 25-50 nm with aninterparticle spacing of 200-300 nm can pin austenite of 30 micronsgrain size in the transfer bars;

(v) Apply rapid cooling between the end of roughing and the start offinish rolling so that (i) the temperature of the transfer bar on entryto finish rolling is below 920° C., the temperature of norecrystallization and (ii) adequate solute niobium >0.03 wt percent, butpreferably 0.04 to 0.05 wt percent is retained for strain accumulationduring finish rolling and transformation hardening on subsequentaccelerated cooling; and

(vi) Control fine austenite grain of about 30 micron size in thetransfer bar to enable thicker strip (17-30 mm) to be produced with lesspancaking in finish rolling compared with heavy pancaking in coarseaustenite grain size of about 60 microns in conventionalthermo-mechanical rolling of higher niobium grades that results inthinner gage.

Advantages of the Present Invention Based on Nano-Scale TiN—NbCComposite Precipitate Engineering for Austenite Grain Size Control:

The foregoing examples are given to demonstrate how nano-scaleprecipitation engineering of TiN—NbC composite precipitates can be usedfor austenite grain size control in upstream processing of austenite toderive benefits in (i) producing thicker gage product (>17 mm) withexcellent strength and fracture toughness at low temperature as measuredby DBTT and DWTT, (ii) obtaining more uniform microstructures, and (iii)minimizing unfavorable crystallographic texture related problems. Theseexamples are for illustrative purposes only and the invention is notintended to be limited to any of the specific examples. However, it willbe understood by those skilled in the art that modifications and changesmay be made to the present invention to combine other elements having ahigh affinity for nitrogen and carbon similar to titanium and niobiumwithout departing from their scope of controlling particle interspacingand size independently to bring about adequate pinning pressure on theaustenite boundary and prevent austenite grain coarsening upstream.

What is claimed is:
 1. A process for controlling austenite grain size inaustenite processing through nano-scale precipitate engineering ofTiN—NbC composites to produce thicker gage product of niobiummicroalloyed steel, comprising: (i) controlling the base chemicalcomposition of a steel product to include Element Amount (wt %) N0.003-0.004 Ti 0.012-0.015 C 0.03-0.07 Nb 0.07-0.15

(ii) lowering the temperature of roughening to end the rougheningoperation in the temperature range of from about 980° C. to 1030° C. toprevent grain refined austenite from coarsening above about 30 micronsby formation of TiN—NbC composite precipitates; (iii) retaining greaterthan about 0.03 wt % niobium in solution in the matrix by rapid coolingof the product to enter the finish rolling operation below thetemperature of no recrystallization, with an austenite grain size ofabout 30 microns; and (iv) applying reduced rolling reduction in thefinish rolling operation to pancake the fine austenite grain size ofabout 30 microns to obtain a sufficient surface to volume ratio toproduce thicker gage resulting steel product.
 2. A process as recited inclaim 1, wherein greater than about 0.04 wt % niobium is retained insolution in the matrix.
 3. A process as recited in claim 1, whereinaustenite grain size is controlled in the range of about 20-40 micronsat entry to the finish rolling operation.
 4. A process as recited inclaim 1, wherein TiN precipitates are in the range of about 10-20 nm andthe inter-particle spacing is about 200-300 nm.
 5. A process as recitedin claim 1 wherein thermodynamic potential for precipitation of NbCoccurs towards the end of the roughing operation at temperatures rangingfrom about 980° C. to about 1030° C.
 6. A process as recited in claim 1,wherein TiN—NbC composites are in the size range of about 20-50 nm.
 7. Aprocess as recited in claim 1, further comprising applying acceleratedcooling upstream between the end of the roughing operation and the startof finish rolling to avoid depletion of solute niobium from the matrixto less than 0.03 wt percent.
 8. A process as recited in claim 7,further comprising applying accelerated cooling upstream between the endof the roughing operation and the start of finish rolling to avoiddepletion of solute niobium from the matrix to less than 0.03 wt percentand enter finish rolling at or below the temperature of norecrystallization.
 9. A process as recited in claim 1, furthercomprising applying accelerated cooling of the steel product to avoidrolling in the partial recrystallization regime and to enter finishrolling below the temperature of no recrystallization.
 10. A process asrecited in claim 1, further comprising controlling nitrogen at or belowabout 40 ppm, and making a titanium addition to meet the stoichiometricrequirement to combine with all nitrogen to form high number density ofTiN precipitate in about the 10-20 nm size range.
 11. A process asrecited in claim 1, further comprising processing the steel product byat least one of conventional plate rolling, conventional hot striprolling, steckel mill rolling, or near net shape processing.
 12. Aprocess as recited in claim 1, wherein the steel product is line pipesteel.
 13. A process as recited in claim 1, wherein the steel product isinfra-structure steel.
 14. A process as recited in claim 1, wherein thesteel product is supermartensitic stainless steel.
 15. A process asrecited in claim 1, wherein the crystallographic texture-relatedanisotropic properties of the resulting steel product are minimized. 16.A process as recited in claim 1, further comprising substitutingtitanium partially or fully in the base chemistry with a member of thegroup consisting of Zr, Hf, Ta, W, V, Cr, Mo, Al and mixtures thereof,each with high affinity for nitrogen to form nano-scale precipitates onwhich NbC can grow epitaxially to give composite precipitates.
 17. Aprocess as recited in claim 1, further comprising partially substitutingniobium in the base chemistry with other microalloying elements withhigh affinity for carbon selected from the group consisting of Zr, Hf,Ta, W, V, Cr, Mo, and mixtures thereof, each to give compositeprecipitates.
 18. A process as recited in claim 1, further comprisingsubstituting solute niobium on entry to finish rolling with otherelements, which exhibit solute drag comparable to niobium.
 19. A processas recited in claim 1, further comprising rapidly cooling the steelproduct to enter finish rolling at a temperature at or below about 920°C.
 20. A process as recited in claim 1, wherein the rolling reduction insaid finish rolling operation is reduced substantially more than 15%.21. A process as recited in claim 1, wherein the steel product exhibitsa gage thickness of about 17-30 mm.
 22. A steel product obtained by theprocess of claim 1.